[LeetCode82]Remove Duplicates from Sorted List II

题目：

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

分类：Linked List

代码：维护两个指针ptr，pre

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        if(!head)
            return head;
        if(head && !head->next)
            return head;
        ListNode *newHead = new ListNode(head->val - 1);
        newHead->next = head;
        ListNode *pre = newHead;
        ListNode *ptr = newHead;
        while(head)
        {
            if(pre->val != head->val && (!head->next || head->val != head->next->val))
            {
                ptr->next = head;
                ptr = ptr->next;
            }
            pre = head;
            head = head->next;
        }
        ptr->next = NULL;
        
        return newHead->next;
    }
};